Pyrolytic carbon may be deposited by thermally decomposing gaseous hydrocarbons or other carbonaceous substances in vaporous form in the presence of a substrate whereupon the deposition will take place. It is well known to coat substrates with layers of pyrolytic carbon for various different purposes. In this respect, the coating may oftentimes completely envelop the substrate, and the composite coated substrate may be the desired end product. In other instances, a very large object or a mandrel may be coated on less than all sides with an extremely thick layer of pyrolytic carbon, and subsequently the mandrel may be machined away or otherwise removed whereby the monolithic coating becomes the desired end product. The present invention is suitable for use with all such instances, even if the underlying substrate is eventually removed.
When pyrolytic carbon is deposited in a fluidized bed apparatus, one of the variables upon which the structure of the pyrolytic carbon will be dependent is the available deposition surface area, relative to the volume of the furnace enclosure wherein the deposition is occurring. Pyrolytic carbon which has a microstructure that is free of growth features will be deposited when the relative amount of deposition surface area is fairly high. Thus, when relatively large objects, for example, objects having at least one dimension equal to 5 mm or more, are being coated, an ancillary bed of small particles (usually of a size measured in hundreds of microns) are included within the furnace enclosure together with the larger object or objects. This arrangement provides sufficient available total surface area to assure that pyrolytic carbon having the desired crystalline form will be deposited. In addition, the random motion of large objects in fluidized beds provide for a relatively uniform deposition of carbon on all surfaces.
However, whenever such submillimeter particles are being coated in a fluidized bed, the total surface area of the particles begins to increase significantly as the diameters of the pyrolytic carbon-coated particles grow. This change in the available deposition surface area in the fluidized bed will result in a change in the physical characteristics of the pyrolytic carbon being deposited if the other coating variables are held constant, e.g., coating temperature, gas flow rate and gas composition; and moreover, when the bed reaches some maximum size, it will collapse and thus limit the thickness of the carbon coating that can be deposited on levitated substrates under constant conditions. Changes in the physical characteristics of the carbon deposited may be undesirable for any of a number of reasons.
It has been found that pyrolytic carbon having good structural strength and uniform physical properties can be deposited as relatively thick coatings upon relatively large objects in the accompaniment of particles if the available fluidized bed surface area is maintained relatively constant by withdrawing particles which have become enlarged in size as a result of coating and feeding smaller size particles into the deposition enclosure. Very generally, the availability of a relatively large amount of deposition surface area in a furnace enclosure of a given volume facilitates the efficient deposition of pyrolytic carbon which is either isotropic or laminar in microstructure and without growth features. In contrast when carbon is deposited on a fixed substrate (e.g., a mandrel) in a chamber without a bed of particles, large gradients in gas composition are established at the gas-solid interfaces where the deposition is occurring, and growth features develop in the microstructure of the deposited carbon. Illustrations and theoretical considerations are reviewed in J. C. Bokros, "The Preparation, Structure, and Properties of Pyrolytic Carbon," in Chemistry and Physics of Carbon, Vol. 5, P. L. Walker (ed.) Marcel Dekker, New York, 1969, Chapter 1.
The crystalline structure, the density and other physical properties, such as the coefficient of thermal expansion, of pyrolytic carbon deposited by the thermal decomposition of a vaporous carbonaceous substance are dependent upon several independently variable operating conditions within the coating apparatus being employed. These conditions include the temperature of the substrate surfaces upon which the deposition is occurring, the overall chemical composition of the atmosphere from which deposition is occurring, the partial pressure of the vaporous carbonaceous substance, the surface area to volume ratio in the active deposition region of the coating apparatus, and the contact time (a parameter based upon the gas flow rate and cross sectional area of the furnace enclosure). Although various of these conditions can be easily regulated and therefore maintained at a constant desired value in many different types of coating apparatus, the surface area to volume ratio is inherently subject to constant change because there is a continuous gradual increase in the total surface area as the items being coated grow in size as the result of the deposition thereupon. When a bed of small spheroids or the like, having an average size between about 50 microns and 600 microns, is present in the active deposition surface region (either because they are the products being coated, e.g., in the case of nuclear fuel particles, or because they are associated with other objects being coated so as to increase the total surface area to void volume ratio), such small particles increase relatively rapidly in surface area as the diameters of these particles grow during deposition of pyrolytic carbon.
A desired surface area to volume ratio is initially provided by starting with an appropriate amount of particles of a particular average size to constitute the fluidized bed. Preferably, an initial surface area to volume ratio is provided near the lower end of the range that produces crystalline pyrolytic carbon having the physical properties desired. Thereafter, as the growing thickness of the deposited pyrolytic carbon layers causes the total surface area to increase, withdrawal of some of the coated particles is initiated so as to decrease the total number of particles to thereby maintain a specific surface area or to regulate its increase in a controlled manner. Thereafter, replacement of the particles being withdrawn with particles of much smaller size is begun.
In coating operations where it is desirable to employ a relatively large surface area to volume ratio, a coating apparatus is of course employed which can maintain such a bed of particles in motion and in association with any larger objects that may also be coated. Examples of suitable coating apparatus of this type include, fluidized bed coaters and rotating drum coaters. A fluidized bed coater such as that of U.S. Pat. No. 3,977,896 issued to Bokros et al. on Aug. 31, 1976 (the disclosure of which is incorporated herein by reference) is an example of one which can satisfactorily perform the pyrolytic carbon deposition process, and hereinafter reference is made to such fluidized bed coaters. See also, U.S. Pat. No., 4,594,2701 issued to Brooks on Jun. 10, 1986, incorporated herein by reference as a further example.
Heretofore mechanical feeding systems have generally been employed to move particles of small size into a fluidized bed coater after or contemporaneous with the removal of particles enlarged by the pyrolytic carbon coating process. Such mechanical feeding systems have generally used moving parts such as gears, screws, disks, shafts, belts and motors. Disadvantageously, such mechanical feeding system parts, when subjected to abrasive, reactive or hard ceramic particles, tend to wear, jam and/or contaminate the particles. Contamination of the feed particles may occur as a result of the introduction of foreign particles which become separated from machine parts as the machine parts wear against one another and against the particles. Such contamination can cause irregularities in the crystal structure of the pyrolytic carbon coating. Irregularities may occur either in the particle coatings or in the coating of a larger substrate being levitated in the fluidized coating bed. Hence, what is needed is a device for feeding particles into a pyrolytic carbon coater that contains no moving or mechanical parts to wear or contaminate the particulate material.
In addition to the above-described features, some particle feeding devices have previously employed a "lost-weight" method of determining whether appropriate amounts of particles have been dispensed into the fluidized bed coater. The "lost-weight" method comprises periodically measuring the weight of a supply of granular material over time and comparing previous measurements with subsequent measurements so as to determine the weight lost from the supply. However, devices that have previously employed the "lost-weight" method have used the "lost-weight" method to control mechanical apparatuses that utilize moving parts to deliver particles into the fluidized bed coater.
U.S. Pat. No. 3,501,097 (hereinafter '097 patent) shows that flowing gas has been used to remove precise amounts of powder from a hopper in a flame spray gun application. Flame spray guns are used to soften fusable material using heat and to project the fusable material onto a surface to which the fusible material is to be applied. The '097 patent shows the use of output conduit backpressure as a feedback mechanism used to regulate the flow of powder in a nozzle. As shown in FIG. 8 of the '097 patent the nozzle 66 is situated beneath the surface of a powder pile 64 and provides constant, but varied flow of gas. The flow of gas moves powder from the powder pile 64 towards the output conduit 60. However, because the '097 patent utilizes varied backpressure as a feedback mechanism, it follows that there will be some pressure variation in the output conduit. This type of system is considered unsuitable for fluidized bed coater applications due to the inherent pressure sensitivity of the pyrolytic carbon coating process.
It is therefore an object of the present invention to provide an improved device for feeding particulate material into a fluidized bed coater that has no mechanical or moving parts that come in contact with the particles being fed.
It is a further object of the present invention to provide a device for feeding particulate material into a fluidized bed coater that allows for the introduction of a constant flow of purge gas into the coater.
It is another object of the present invention to provide a device for feeding particulate material into a fluidized bed coater that includes a feedback apparatus that controls the introduction of predetermined amounts of particulate material into the fluidized coating bed.
These and other objects of the present invention will become evident from a reading of the following description in conjunction with the accompanying drawings.